The present invention relates generally to semiconductor manufacturing and specifically to a method and system for polishing a semiconductor wafer.
In the fabrication of integrated circuits (ICs), it is often necessary to polish a thin flat wafer of semiconductor material. In general, a semiconductor wafer is polished in order to provide a planarized surface. This polish serves to remove topography of a deposited film generated by underlying layers during IC fabrication as well as surface defects such as scratches, roughness, embedded particles, or crystal lattice damage. This polishing process is often referred to as chemical mechanical planarization (CMP) and is utilized to improve the quality and reliability of semiconductor integrated circuits. The CMP process is usually performed during the formation of various devices and integrated circuits on the wafer.
In general, the chemical mechanical planarization process involves holding a wafer under down force against a moving polishing pad. The polishing pad is adhered typically on top of a rotating platen and wetted with slurry. The slurry contains abrasives such as alumina or silica, and chemicals to effect easier removal of the material being polished, and attain a surface as smooth and defect-free as possible. A rotating polishing head or wafer carrier is typically utilized to hold the wafer and apply pressure during polish. A backing film is optionally positioned between the wafer carrier and the wafer to improve polish uniformity.
CMP processes have been used to polish surfaces that are made of silicon oxide, silicon nitride, tungsten, aluminum, copper, etc. Inherent imperfection and a degree of unrepeatability in the process cause non-uniformity in the removal amount or post-polish film thickness. The process parameters to be adjusted in a CMP process include, for example, polish time, platen speed, head speed, slurry flow rate, and applied force. The applied force can be applied on some current polishers over many zones across the wafer. By adjusting the process parameters, a highly non-uniform process can be tuned to produce improved uniformity. Conventionally, such process tuning is done on a post-priori, or ex-situ, manner, i.e., by measurements of film thickness and uniformity after the polish process is completely finished to determine the process profile, and then adjusting process parameters for improvement of the result if desired. There is no available technique that exists today that will allow the semiconductor manufacturer to measure, obtain a feedback, and adjust and control process parameters during the polish.
Conventional polishing methods utilize an optical endpoint detector to track the thickness of the wafer during the polishing process. The technology of optical endpoint detection is described in the following publications: (1) N. E. Lustig et al., xe2x80x9cIn-situ endpoint detection and process monitoring method and apparatus for chemical-mechanical polishing,xe2x80x9d U.S. Pat. No. 5,433,651 (1995); (2) Y. Ushio et al., xe2x80x9cIn-situ monitoring of CMP process utilizing 0-order spectrometry,xe2x80x9d Proceedings of CMP-MIC Conference, 23 (1999).
The optical endpoint detector utilizes an optical signal to sample the thickness of transparent film on a wafer during the wafer""s relative motion across an endpoint window in the polishing pad. The optical endpoint operates by impinging light from a source below the pad through the endpoint window at a wafer being polished and detecting the light reflected from the wafer. By observing optical interference, the film thickness of transparent materials can be determined. The endpoint is achieved when the film thickness reaches the targeted value.
When the material being removed is opaque rather than transparent, such as for metal films (tungsten, copper, etc.), a dramatic change in the intensity of the reflected light is observed when the polished film is almost cleared or cleared and the underlying film (normally a dielectric material such as silicon oxide, silicon nitride) is exposed. Thick metal films have high reflectance, where as the dielectric material have lower reflectance. If the impinging light has many wavelengths, a change of color of the film as it is almost cleared is also observed, signaling that endpoint is reached.
For a further description of the conventional methodology, please refer to FIG. 1. FIG. 1 is an illustration of the conventional polishing process. For purpose of easier understanding, a linear polisher is illustrated. The optical endpoint signal samples the film thickness of the wafer 10 through the endpoint window 12. For this type of polisher, the sample is taken once per pass of the endpoint window, the signal being averaged over areas from one edge of the wafer to the other edge. (Here for a simplified discussion, we have neglected wafer rotation, and the two wafer edges where the endpoint window enters and exits underneath the wafer are opposite each other. In general, they are not on the same diameter line through the center.) As such, no spatial resolution is obtained to ascertain the polish profile (e.g., edge fast or edge slow) during polish. Post-polish measurements are required to obtain this polish profile and before any process adjustment can be made. Such method of control is ex-situ, requiring human intervention
Accordingly, what is needed is a more effective method and system for sampling the thickness of film layers on wafers that facilitates in-situ adjustments of the polish parameters during process. The method and system should be simple, cost effective, and adaptable to existing technology. The present invention addresses such a need.
The present invention provides a method and system for polishing a wafer surface. The method and system comprises determining whether a thickness of the wafer surface is uniform while the wafer surface is being polished, and adjusting the polishing process while the wafer surface is being polished based on the determination of whether the thickness of the wafer surface is uniform.
Through the use of the method and system in accordance with the present invention, in-situ adjustments can be made to the CMP polishing process while the wafer is actually being polished. This results in a substantial improvement in polishing uniformity.